Method of mine backfilling and material therefor

ABSTRACT

A method of backfilling in underground mine operations by a mill tailings slurry which is dewatered to provide a material with a controlled water content to permit the dewatered material to be used as a backfill without requiring any substantial dewatering after placement and which material can, when desired, be mixed with selected quantities of cement to provide a mortar for backfilling or surface covering.

In mining it is well known to backfill excavated or mined out regions ofan underground mine by transporting a slurry of sands or mill tailingshaving 40 to 70 percent solids, by weight, to the stope areas to befilled. Once in the stope the transport water of the slurry createsserious problems as the excess transport water in the backfill causesthe backfill to have the characteristics of a large hydrostatic bodywhich can exert a high, and sometimes excessive, pressure on thebackfill retaining structures. Further the backfill is not utilizable inthe mining operations until it has a consistency and strength to permitmen and machines to be safely supported on the surface of the fill. Atpresent such excess transport water is removed by elaborate decantingand draining operations which have presented problems is providing thedewatering structures, in cleaning the accumulations of slimes on thedewatering structures and the disposing of the removed excess water fromwithin the mine. In addition, the rate of dewatering of prior slurriesvaries due to various factors such as the permeability and location ofthe fill such that men and machines are not supportable by thedewatering fill for a period of time which can vary from a day to acouple of weeks. Obviously such time delay can adversely effect the mineproduction. Other obvious problems are created when such slurries spillover or break the retaining structures.

It is also common practice to add cement to slurries prior totransporting the slurry to obtain a higher strength backfill. With thehigh percentage of transporting water a large percentage of the cementis retained in the water and accumulates in water pockets in the fill orruns off with the drain water so that the strength advantage for theamount of cement added is not obtained.

Still other prior hydraulic backfill systems have classified the milltailings prior to forming the slurry to remove a selected percentage ofthe fine particles from the tailings, such as removing most of theparticles smaller than 200 mesh, in order to provide a slurry in thenature of 40 to 70 percent solids by weight which is easier to dewaterin place and reduce the expense associated with cleaning up slimes. Suchsystems obviously require the fines to be removed at the surface,however, slurries without such fine particles have other seriousdisadvantages in that transporting of larger particles causes higherabrasion in the transport system and higher friction losses since thefines content of a slurry functions as a transporting lubricant. Aparticular disadvantage in using classified tailings in a slurry is thatmore consolidation occurs with large particles than occurs with a slurrycontaining fines since classified tailings are poorly graded in that thesize of particles are more uniform and unclassified tailings are wellgraded in that all sizes of particles are contained therein.

The various disadvantages of the prior backfilling techniques arecharacterized by the disadvantage that the method of mining and the rateof mining is controlled by the backfilling method employed. Also priorhydraulic backfilling systems have not proven practical for some miningmethods.

The present invention employs a method of backfilling by dewatering aslurry of mill tailings after hydraulic transport into the mine butbefore placement of the fill material to produce a material having acontrolled water content such that extensive dewatering of in-place fillis not required to quickly obtain a surface capable of supporting menand material. Also by mixing cement with the dewatered material theresultant material-cement mixture is a mortar that can be placed asdesired without requiring subsequent dewatering. The mortar produced bythe method of this invention is of a consistency of the mortar as iscommonly employed in the building trades, however, it is not of the samecomposition as the mortar as used in the building trades. In addition,the present method utilizes unclassified mill tailings so that theresultant backfill material has a higher strength than dewateredmaterial not including the fine particles. The method of the presentinvention also provides a backfill which is able to support men andmachinery almost shortly after backfilling has been completed and whichis not subject to reliquidification subsequent to placement.

Accordingly, one object of this invention is to provide a new andimproved method of backfilling for underground mining operations.

Another object of this invention is to provide a new and improvedmaterial for use in backfilling in underground mining operations.

A more specific object of this invention is to provide a new andimproved method of backfilling for underground mining operationsutilizing mill tailings having a selected water content with whichcement is mixed to provide a mortar.

Still another more specific object of this invention is to provide a newand improved method of backfilling for underground mining operationsutilizing a mill tailings slurry in which the percentage of waterremoved is controlled to provide a material which can be used directlyfor backfilling.

These and other objects of this invention will become better understoodupon consideration of the following detailed description of the methodand material as is presently preferred with relation to one type ofmining system shown in the drawing.

The drawing is a general schematic illustration of a typical mineemploying a "cut and fill" mining system; however, it will beappreciated that mining systems follow bedded ore deposits as they occurin nature and therefore are not necessarily of the precisenessillustrated in the drawing. Backfilling is extensively used in "cut andfill" mining and accorgingly well illustrates a mining system to whichthe advantages of this invention are applicable. In the schematicillustration of a "cut and fill" mine a vertical main access shaft 14extends downwardly from the earth's surface 16 to provide access to anunderground in situ mineral ore deposit 18 via a plurality of verticallyspaced drifts 20, only one of which is shown, extending generallyhorizontally from shaft 14 through the deposit 18 to define a respectiveplurality of vertically spaced levels in the mine. The drift at thelowermost level shown in the figure provides a main haulageway 22 whichreceives extracted ore from one or more mine levels thereabove via aplurality of suitably vertically extending ore passes 24 extendingbetween the main haulageway and the various mining locations or stopes26 from one or more of the overlying drifts 20.

In the conventional "cut and fill" mining the minieral ore is mined inany suitable manner such as by known mining machines or known drillingand blasting techniques from the upper and side portions of the stope 26to obtain a quantity of broken or extracted ore 28 on the floor of stope26 which mined ore is then transported in any suitable manner, such asby a conventional dragline slusher 30 as illustrated, into one of theore passes 24 for subsequent gravitational delivery thereof tohaulageway 22. As mining progresses the removal of ore continuallyrelocates the upper portion of the stope 26 upwardly within the oredeposit 18 such that support must be provided for the walls of the stope26 formed during such mining which were previously supported by the orethat has been removed. Accordingly, mining at any particular stope 26location must be periodically interrupted for backfilling with a desireddepth of fill material to elevate the floor of stope 26 to permit miningto continue.

Inasmuch as the mine layout for "cut and fill" mining and various miningmethods may be employed to mine the ore deposit 18 as are well known inthe art further description thereof is not necessary to theunderstanding of this invention. It is also well known that theextracted ore prior to the removal of its metals constituents, iscrushed and ground such that after the metals have been extracted thenonmetallic constituents are a waste by-product which is primarily afinely ground rock commonly referred to as mill tailings. As is alsoknown such mill tailings are mixed with water in a slurry preparationmeans 32 to provide a slurry which slurry is pumped by a suitable pump34 through a conduit 36 to the desired location for use within the mine.Although particle sizes in mill tailings vary due to the various metalextraction processes employed, a typical sample of mill tailings will beof minus 28 mesh having particles of 600 microns to submicron sizetherein. Further the water content, and obviously the solids content ofthe slurry will also vary dependent upon the parameters required to pumpthe slurry to its desired use location. With the method of thisinvention the incoming slurry is dewatered by a centrifugal separator 38which is of a type to produce a material of a composition and/orconsistency as described hereinafter at a rate to permit the filling ofthe open area of the stope 26 at a rate consistent with the miningmethod desired.

Although the slurry is specified as being dewatered herein to providethe material of this invention it is to be understood that all water isnot removed from the incoming slurry and that the material dischargedfrom the centrifuge has a water content which water content iscontrolled to provide a material of the proper consistency. A centrifugeof a structure and which is operable to provide such material dischargeis more fully described, shown and claimed in copending patentapplication, shown and claimed in copending patent application Ser. No.720,200 the disclosure of which is incorporated herein for a betterunderstanding of this invention. With the method of this invention theslurry is dewatered regardless of mill tailing. The water content of theslurry is selected with reference to the composition of mill tailings tobe transported, the mechanical equipment available for such transportingand the plysical system used to transport the slurry. Basically in agiven situation only the water necessary to provide the most efficiencytransportation is used in the slurry; however, the water content of theslurry is not critical as long as sufficient water is used to permit themill tailings to be transported to the desired location. A slurry watercontent of 30 to 70 percent by weight is satisfactory for transportingthe slurry used in this invention. Also it is not essential in thisinvention that each and every solid particle be removed from the slurry;however, the solids content of the discharge from the separator 38 isheld to a practical minimum such as 5 percent by weight due to theoperating parameters of a centrifuge as more fully set forth inapplication Ser. No. 720,200. Preferably the moist mill tailingsdischarge is mixed with a suitable quantity of cementituous material ina suitable mixer 44 to provide a mortar for backfilling. Thereafter suchmortar is placed as desired in stope 26 in any suitable manner such asby a mortar pump 48 the discharge of which is deposited upon the top ofthe prior backfill between the walls of the stope 26 therebelow. Once inthe stope 26 the mortar can be spread as desired by known devices suchas the equipment used for spreading concrete. In other instances it isdesirable to place the mortar overhead and/or on vertical walls withinthe mine which can be accomplished with suitable equipment such as thatused in the shotcrete process. Thus, for the purposes of this inventionthe mortar is deposited at a consistency such that the mortar need notbe supported to permit dewatering.

With this invention it is not essential that the mortar in place be voidof any excess water since certain quantities of water can be handled bythe known mine drainage systems. What is required is that the watercontent of the mortar be sufficiency low to permit the mortar to remainin place and stabilize without requiring the erection of supportingbulkheads or dewatering structures. Consequently the water content ofthe material discharged from the separator 38 must be controlledinasmuch as the water content of such material will determine,disregarding the water absorbed during hydration by the added cement,the water content of the resultant mortar. Experimentation hasestablished that the material discharged from the separator 38 wouldhave a water content not in excess of 17% to 25% by weight, with about20% being preferable. Such 20% water content permits the material to bedischarged continuously from the separator 38 as a flowable homogeneousmass. The amount of cement added to the discharge from the separator 38can be varied as desired with the addition of a higher percentage ofcement providing a higher strength mortar at an increased cost. Forbackfill purposes a 3% by weight addition of cement provides asatisfactory mortar. In instances where the mortar is placed on walls upto 20% by weight can be added and provide a mortar which, uponhardening, has a higher strength.

With such water content the mortar can be handled in the manner in whichcements are presently handled both in placing the mortar and in workingthe mortar. Since the cement added is thoroughly mixed into the materialdischarged from the centrifuge there is a uniformity of mortar such thatthe resultant hardened material is uniform and capable of supporting menand machines without encountering soft or mushy areas. By mixing thecement with the discharged material the cement coats the material toprevent cement migration as occurs when cement is added to a milltailings slurry prior to transport. In particular by using unclassifiedtailings the entire range of particle sizes are utilized within themortar with the primary advantage residing in the utilization of thefine particle to provide a higher density material. Such higher densityis achieved since the fine particles become located between largerparticles rather than having voids between larger particles. Voidsbetween particles when under load, such as when machinery transversessuch materials, crush the larger particles and permit the largerparticles to disintegrate and fill such voids. Such void filling doesprovide a consolidated material; however, the surface of the materialbecomes uneven during use. Further until complete consolidation occursthe less dense material does not have the same homogeneousness as themortar of this invention. By mixing the cement after dewatering thecement is dispersed throughout the mortar which dispersement coupledwith the lower water content of the mill tailings discharge from theseparator 38 provides a substantially higher strength backfill than thestrength of the backfill of prior methods utilizing the same amount ofcement. Alternatively, by so adding and mixing the cement less cement isused to obtain the same strength of backfill as is obtained with priormethods. As is known, cement hydrolyzes and during hydrolyzation absorbswater; accordingly, the adding of cement as described above reduces thewater content of the mortar. Depending upon the resultant mortarstrength desired the quantity of added cement is selected as desired,with reference to the free water available in the mortar. Thus, with themethod of this invention by controlling the water content of thedischarge from the separator 38 a mine operator can obtain differentbackfills having various structural characteristics to provide a widelatitude as to the manner in which the mortar can be used. Such latitudepermits the mine operator to select a mining system dependent upon thematerials produced by this invention rather than a system controlled bythe laws of hydraulics. An operator can also control the water contentof the mortar by adding additional cement in instances where there maybe an excess of free water in the material discharged from thecentrifuge. The mine operator has even more latitude as to theutilization of the mortar of this invention by using the variousadmixtures known in the use of the cements such as accelerators andcuring aids.

The liquid discharge from the separator 38 is transported to a suitablelocation for disposal as is known in the art. As shown, the liquiddischarge is preferably returned to the surface by suitable meansincluding a conduit 37 for discharge into a settling tank 39 to providewater for mixing with mill tailings as previously described. Settlingtank 39 is preferred so that the solids content of the liquid dischargefrom the separator 38 can be easily separated from the return water.

Although the preferred embodiment of this invention utilizes cement theaddition of cement is not essential to obtain a backfill materialsuperior to the backfill materials presently being used in undergroundmining since the material discharged from the centrifuge has a low watercontent compared with the water content of slurries which are notdewatered prior to backfilling. Thus, as indiciated, with a watercontent of from 17% to 25% by weight in the material discharged suchmaterial can be used directly for backfilling without requiring theextensive dewatering and bulkheading presently employed. When cement isnot to be added the mixer 44 is not employed. The advantages of usingunclassified tailings, higher density of backfill, eliminating elaboratedewatering and bulkhead structures, and a shorter time period until thebackfill can be utilized compared to present methods are still obtained;however, not necessarily to the same degree as is obtained by addingcement to the material discharged from the centrifuge.

Although a cut and fill mining system has been described the method andmaterial of this invention are equally applicable to other miningsystems such as the longwall systems utilized in South Africa at thepresent time. In such longwall systems the mined seam is backfilled toprovide roof support for the miners at the face. Prior slurries, whichare dewatered in place, present serious problems in providing adequateroof support which is minimized by the material and method of thisinvention due to the higher density of backfill material and the lowershrinkage of the material of this invention. In addition, the problemsattendant with excess water in longwall seams are eliminated.

Although a preferred embodiment of the invention as presentlycontemplated has been described and shown and modifications thereof havealso been described, those skilled in the art to which this inventionrelates will readily discern that the methods and materials of thisinvention can be utilized in numerous ways without departing from thespirit and scope of this invention as set forth in the following claims.

What is claimed is:
 1. A method of backfilling underground cavities andthe like comprising: transporting a slurry of mill tailings and waterthrough a closed conduit system to an underground location, dewateringsaid slurry at said location as said slurry is received thereat toproduce a material of a consistency that said material stabilizes assubstantially an unsupported mass without removing water therefrom byauxiliary means, and placing said material in a cavity or the like to befilled.
 2. The method of claim 1 wherein said transporting is for anextended period of time and said dewatering is continuous throughoutsaid transporting.
 3. The method of claim 1 wherein said slurry has awater content in the range of 35 to 70 percent by weight and saidmaterial has a water content of no more than 25 percent by weight andpreferably has a water content of about 20 percent.
 4. The method ofclaim 1 with the additional step of adding cement in a selected quantityto said material prior to said placing.
 5. The method of claim 1 whereinsaid mill tailings are unclassified.
 6. The method of claim 4 whereinsaid mill tailings are unclassified.
 7. The method of claim 4 whereinsaid material is a mortar having a strength dependent upon said quantityof cement.
 8. The method of claim 1 wherein the water content of saidmaterial is substantially 17 percent by weight.